Photosensitive letterpress printing original plate

ABSTRACT

The present invention provides a photosensitive letterpress printing original plate provided with a heat sensitive mask layer having high light blocking effect and durability yet prepared as a thin film. A photosensitive letterpress printing original plate including at least (A) a supporting member, (B) a photosensitive resin layer, and (C) a heat sensitive mask layer that are successively laminated, wherein the heat sensitive mask layer (C) contains carbon black and, as a dispersion binder therefor, a butyral resin as well as polyamide containing polar group selected from the group consisting of polyamide containing a tertiary amine group, polyamide containing a quaternary ammonium salt group, polyamide containing an ether group and polyamide containing a sulfonic group.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a photosensitive letterpress printing originalplate to be used for producing a letterpress printing plate according toa Computer to Plate Technique, and in particular, it concerns aphotosensitive letterpress printing original plate provided with a heatsensitive mask layer having high light blocking effect and durabilityeven if prepared as a thin film.

BACKGROUND ART

In recent years, in the field of letterpress printing, a Computer toPlate Technique (CTP Technique) , known as a digital image-formingtechnique, has been widely used as a general technique. The CTPtechnique is a method in which information processed on a computer isdirectly outputted onto a printing plate so that a concave/convexpattern to forma relief is obtained. This technique eliminates thenecessity of negative film producing processes, thereby making itpossible to reduce costs and time required for forming the negativefilm.

In the CTP technique, the negative film, conventionally used so as tocover areas that should not be polymerized, is replaced by a mask thatis formed and integrated within a printing plate. With respect to themethod for obtaining the integration mask, a method has been widely usedin which an infra-red ray sensitive layer (a heat sensitive mask layer)that is opaque to chemical rays is formed on a photosensitive resinlayer and by evaporating this infra-red ray sensitive layer by using aninfrared laser, an image mask is formed (cf. Japanese Patent ApplicationRepublication No. 506201/95).

As the heat sensitive mask layer, those layers, made from carbon blackserving as an opaque material to radiations and a binder, are generallyused. The heat sensitive mask layer is subjected to abrasion by aninfrared laser. Therefore, the layer is desirably made thinner from theviewpoint of abrasion efficiency. Here, as the layer is made thinner,influences of the caused wrinkles onto the relief become lesser.However, in order to prevent chemical radiations from being transmittedinto a photo-polymerizable layer, the photosensitive mask layergenerally needs to have a transmittance optical density (light blockingeffect) of 2.0 or more. An ideal heat sensitive mask layer is such alayer as to provide a predetermined transmittance optical density (lightblocking effect) or more yet prepared as a thin film.

In general, the optical density (light blocking effect) of the heatsensitive mask layer is defined by the following formula.

Optical density=log (100/T)=εcl

Here, T represents a transmittance (%), c represents a density of aninfra-red ray absorbing substance (mol·l⁻¹), l represents a thickness(cm), and ε represents a molecular extinction coefficient(l·mol⁻¹·cm⁻¹). As can be understood by the above-mentioned formula, theoptical density can be enhanced by increasing the density of theinfrared-ray absorbing material (carbon black) ; however, in this case,the film becomes too brittle, which causes innumerable scratches inworking process. The scratched portions fail to shield light, therebycausing unnecessary images to be formed on a relief. Moreover, althoughthe optical density can be increased by making the coat film thicknessthicker; however, in this case, high energy is required to causeabrasion, and wrinkles tend to be caused on the relief . Wrinkles on therelief become in particular a serious problem in the case of a waterdeveloping plate whose photosensitive resin layer is flexible.

Here, it has been known that the light blocking effect is changeddepending on the dispersing state of carbon black even when the carbonblack density and the layer thickness are the same. In general, thebetter the dispersing state, the higher the optical density becomes.Here, the dispersing state of carbon black is highly influenced by abinder resin.

Up to now, as the binder resin for the heat sensitive mask layer, anylon resin (cf. Japanese Patent Application Laid-Open (JP-A) No.305030/96) , a polyvinyl alcohol resin (cf. Japanese Patent ApplicationLaid-Open (JP-A) No. 171247/97), a heat decomposable polymer (cf.Japanese Patent Application Laid-Open (JP-A) No. 2002-214792) or aplurality of polymers have been used. In the case where the nylon resinis used, although the durability of the heat sensitive mask layerbecomes superior, the dispersing state of carbon black becomesundesirable, causing the necessity of increasing the layer thickness soas to achieve a predetermined optical density. On the other hand, in thecase where the polyvinyl alcohol resin is used, the resulting defect isthat the durability of the film deteriorates. In the case where the heatdecomposable polymer is used as well, it is not possible to obtain asatisfactory abrasion efficiency. Therefore, at present, a heatsensitive mask layer having a high optical density (light blockingeffect) with superior durability yet prepared as a thin film has notbeen achieved.

DISCLOSURE OF THE INVENTION Problem that the Invention is to Solve

The present invention has been devised so as to solve the problems withthe conventional art, and its objective is to provide a photosensitiveletterpress printing original plate having a heat sensitive mask layerthat has high light blocking effect and durability yet prepared as athin film.

Means for Solving the Problem

As a result of intensive studies made to achieve the above-mentionedobjective, the inventors consequently have noted that, by using abutyral resin and polyamide containing polar group as a binder forimproving the dispersion property for carbon black of a heat sensitivemask layer, desired effects can be obtained, thereby completing thepresent invention.

That is, the present invention relates to a photosensitive letterpressprinting original plate including at least (A) a supporting member, (B)a photosensitive resin layer, and (C) a heat sensitive mask layer thatare successively laminated, wherein the heat sensitive mask layer (C)contains carbon black and, as a dispersion binder therefor, a butyralresin as well as polyamide containing polar group selected from thegroup consisting of polyamide containing a tertiary amine group,polyamide containing a quaternary ammonium salt group, polyamidecontaining an ether group and polyamide containing a sulfonic group.

In one preferable mode of the photosensitive letterpress printingoriginal plate of the present invention, the butyral resin and thepolyamide containing polar group have a weight ratio in a range from 20to 80:20 to 80, and the carbon black and dispersion binder have a weightratio in a range from 25 to 50:20 to 75.

Advantages of the Invention

The photosensitive letterpress printing original plate of the presentinvention makes it possible to improve the dispersing property of carbonblack by using a butyral resin as a dispersion binder for carbon blackof the heat sensitive mask layer and also to enhance the durability aswell as improving the dispersing property due to the polar group byusing polyamide containing polar group as a binder for carbon black.Therefore, the present invention makes it possible to achieve a highlight blocking effect by using the heat sensitive mask layer with a thinthickness, and consequently to provide a photosensitive letterpressprinting original plate that can be subjected to abrasion at low energy,and is less vulnerable to wrinkles on the relief.

BEST MODE FOR CARRYING OUT THE INVENTION

The photosensitive letterpress printing original plate of the presentinvention will now be illustrated in detail as follows.

The photosensitive letterpress printing original plate of the presentinvention has a constitution in which at least (A) a supporting member,(B) a photosensitive resin layer, and (C) a heat sensitive mask layerare successively laminated.

The supporting member (A) to be used for the original plate of thepresent invention is flexible, and preferably made from a materialhaving a superior dimension stability, and examples thereof include: asupporting member made of metal, such as steel, aluminum, copper andnickel, and a supporting member made of a thermoplastic resin, such as apolyethylene terephthalate film, a polyethylene naphthalate film, apolybutylene terephthalate film and a polycarbonate film. Among these,the polyethylene terephthalate film, which has a superior dimensionstability and a sufficiently high viscoelasticity, is in particularpreferably used. The thickness of the supporting member is set to 50 to350 μm, preferably, to 100 to 250 μm, from the viewpoints of mechanicalproperties, shape stability and handling characteristics upon producinga printing plate. Moreover, if necessary, in order to improve anadhesive property between the supporting member and a photosensitiveresin layer, an adhesive may be formed between them.

The photosensitive resin layer (B) to be used for the original plate ofthe present invention is composed of essential components, such as asynthetic polymer compound, a photopolymerizable unsaturated compoundand a photopolymerization initiator, and desirable additives, such as aplasticizer, a thermal polymerization inhibitor, a dye, a pigment, anultraviolet-ray absorbing agent, perfume, and an antioxidant.

As the synthetic polymer compound, conventional soluble syntheticpolymer compound may be used, and examples thereof include: polyetheramide (cf. Japanese Patent ApplicationLaid-Open (JP-A) No. 79437/80etc.), polyetherester amide (cf. Japanese Patent Application Laid-Open(JP-A) No. 113537/83 etc.), polyamide containing a tertiary nitrogenatom (cf. Japanese Patent Application Laid-Open (JP-A) No. 76055/75etc.), polyamide containing a tertiary nitrogen atom of ammonium salttype (cf. Japanese Patent Application Laid-Open (JP-A) No. 36555/78 etc), addition polymer of amide compound having one or more amide bond andorganic diisocyanate compound (cf. Japanese Patent Application.Laid-Open(JP-A) No. 140737/83 etc.), and addition polymer of diamine having noamide bond and organic diisocyanate compound (cf. Japanese PatentApplication Laid-Open (JP-A) No. 97154/92 etc.). Among these, thepolyamide containing a tertiary nitrogen atom and the polyamidecontaining a tertiary nitrogen atom of ammonium salt type arepreferable.

As the photopolymerizable unsaturated compound, ring-opening additionreaction product of polyglycidyl ether of polyhydric alcohol,methacrylic acid and acrylic acid is exampled. As the polyhydricalcohol, dipentaerythritol, pentaerythritol, trimethylolpropane,glycerin, ethylene glycol, diethylene glycol, triethylene glycol, andethylene oxide addition product of phthalic acid are exampled. Amongthese, trimethylolpropane is preferable.

As the photopolymerization initiator, benzophenones, benzoins,acetophenones, benzyls, benzoin alkylethers, benzyl alkylketals,anthraquinones, and thioxanthenes are exampled. Specifically,benzophenone, chlorobenzophenone, benzoin, acetophenone, benzyl, benzoinmethylether, benzoin ethylether, benzoin isopropylether, benzoinisobutylether, benzyl dimethylketal, benzyl diethylketal, benzyldiisopropylketal, anthraquinone, 2-ethylanthraquinone,2-methylanthraquinone, 2-allylanthraquinone, 2-chloroanthraquinone,thioxanthone, and 2-chlorothioxanthone are exampled.

The heat sensitive mask layer (C) to be used for the original plate ofthe present invention is composed of carbon black that is a materialhaving a function for absorbing infrared laser rays to convert them intoheat and a function for blocking ultraviolet rays, and a dispersionbinder used for the carbon black. Moreover, desired components exceptfor these, such as a pigment dispersant, a filler, a surfactant or acoating aid, may be contained therein within such a range as not toimpair the effects of the present invention.

The present invention is characterized in that a butyral resin andpolyamide containing polar group are used in combination as a dispersionbinder for carbon black. The butyral resin can improve the dispersingproperty of the carbon black and contribute to high light blockingeffect; however, the use of the butyral resin alone makes the coat filmof the heat sensitive mask layer brittle to cause deterioration ofdurability. Therefore, in order to maintain the durability, the presentinvention uses the polyamide containing polar group. The polyamidecontaining polar group is superior in the dispersing property of carbonblack because of the effect of the polar group contained therein, andcauses no deterioration of the dispersing property even when one portionof the butyral resin is replaced by this. Here, both of the butyralresin and the polyamide containing polar group are dissolved in alcoholand water; therefore, when these are concomitantly used as dispersionbinders, it becomes possible to easily produce a heat sensitive masklayer that is superior in handling upon forming a film as well as indeveloping property by a solvent and water. Since the heat sensitivemask layer, made of the butyral resin and the polyamide containing polargroup, is easily dissolved in water, it is in particular preferably usedas a heat sensitive mask layer for water developing plates.

The butyral resin to be used for a dispersion binder is also referred toas polyvinyl butyral, and one kind of polyvinyl acetal produced byallowing polyvinyl alcohol and butyl aldehyde to react with each otherby using an acid catalyst.

The polyamide containing polar group to be used as the dispersion binderis selected from the group consisting of polyamide containing a tertiaryamine group, polyamide containing a quaternary ammonium salt group,polyamide containing an ether group and polyamide containing a sulfonicgroup.

The weight ratio of the butyral resin and the polyamide containing polargroup in the dispersion binder of the heat sensitive mask layer (C) ispreferably set in a range from 20 to 80:20 to 80. When the weight ratioof the two components is not within the above-mentioned range, there isa fear that the dispersing property of carbon black and the durabilityof the coat film of the heat sensitive mask layer might not be achievedin a well-balanced manner.

The weight ratio of the carbon black and the dispersion binder in theheat sensitive mask layer (C) is preferably set in a range from 25 to50:20 to 75. When the weight ratio of the two components is not withinthe above-mentioned range, there is a fear that the light blockingeffect due to carbon black might not be achieved when the heat sensitivemask layer is applied as a thin film.

The heat sensitive mask layer (C) is preferably designed to have anoptical density of 2.0 or more, more preferably, an optical density of2.0 to 3.0, and most preferably, an optical density of 2.2 to 2.5,relative to chemical rays.

The layer thickness of the heat sensitive mask layer (C) is preferablyset to 0.5 to 2.5 μm, more preferably, to 1.0 to 2.0 μm. When the layerthickness is set to the above-mentioned lower limit or more, it ispossible to obtain an optical density of not less than a predeterminedvalue, without the necessity of a high coating technique. Moreover, whenthe layer thickness is set to the above-mentioned upper limit or less,high energy is not required for evaporation of the heat sensitive masklayer, thereby making it possible to provide an advantageous method fromthe viewpoint of costs.

It is preferable to install a peelable flexible cover film on the heatsensitive mask layer (C) to protect the printing original plate.Examples of the preferable peelable flexible cover film include apolyethylene terephthalate film, a polyethylene naphthalate film and apolybutylene terephthalate film. However, such a protective film is notabsolutely necessary.

Although the method for manufacturing a letterpress printing originalplate of the present invention is not particularly limited, it isgenerally manufactured by using the following processes:

First, components of the heat sensitive mask layer, such as a binder,except for carbon black are dissolved in an appropriate solvent, andcarbon black is dispersed therein to prepare a dispersion solution.Next, this dispersion solution is applied to a supporting member (forexample, a PET film) for a heat sensitive mask layer, and the solvent isevaporated so that one of laminated bodies is formed. In the meantime,in a separated manner from this, a photosensitive resin layer is formedon a supporting member by a coating process so that the other laminatedbody is prepared. The two laminated bodies, thus obtained, are laminatedunder pressure and/or a heating process so that the photosensitive resinlayer is arranged adjacent to the heat sensitive mask layer. Here, thesupporting member for the heat sensitive mask layer functions as asurface protective film after completion of the printing original plate.

When the protective film is present, a method for manufacturing aprinting plate from the printing original plate of the present inventionincludes processes in which, first, the protective film is removed fromthe photosensitive printing original plate. Thereafter, the heatsensitive mask layer is imagewise-irradiated with an IR laser so that amask is formed on the photosensitive resin layer. Preferable examples ofthe IR laser include an ND/YAG laser (1064 nm) and a diode laser (forexample, 830 nm) . A laser system desirably used for the Computer toPlate Technique is commercially available, and, for example, a CDI Spark(made by Esko-Graphics Co., Ltd.) may be used. This laser systemincludes a rotation cylinder drum used for holding a printing originalplate, an IR laser irradiating device and a layout computer, and imageinformation is directly transferred from the layout computer to thelaser device.

After the image information has been written in the heat sensitive masklayer, active light rays are applied onto the entire surface of thephotosensitive printing original plate through the mask. This processmay be carried out with the plate attached to the laser cylinder;however, a method is generally used in which, after the plate has beenremoved from the laser cylinder, the irradiation process is carried outby using a commonly-used irradiation unit having a flat plate shapebecause this method is more advantageous in that even a plate size outof the standard can be appropriately handled. Examples of the activelight rays include: ultraviolet rays having a wavelength in a range from150 to 500 nm, in particular, from 300 to 400 nm. Examples of its lightsource include: a low-pressure mercury lamp, a high-pressure mercurylamp, a super-high pressure mercury lamp, a metal halide lamp, a xenonlamp, a zirconium lamp, a carbon arc lamp and an ultraviolet-rayfluorescent lamp. Thereafter, the irradiated plate is developed so thata printing plate is obtained. The developing process can be carried outby using a commonly-used developing unit.

EXAMPLES

The present invention will now be specifically illustrated by way of thefollowing Examples although the present invention is not limitedthereto.

Preparation of the Dispersion Binder

As the dispersion binder, a butyral resin, polyamide containing atertiary amine group, polyamide containing an ether group, polyamidecontaining a quaternary ammonium salt group, polyamide containing asulfonic group, polyamide containing no polar group and polyvinylalcohol were prepared. BM-5 made by Sekisui Chemical Co. , Ltd. was usedas the butyral resin. With respect to polyamide containing a tertiaryamine group, the polyamide containing an ether group, the polyamidecontaining a quaternary ammonium salt group and the polyamide containinga sulfonic group, those materials synthesized as described below wereused. Macromelt 6900 made by Henkel Japan Ltd. was used as the polyamidecontaining no polar group. GH23 made by Nippon Synthetic ChemicalIndustry Co., Ltd was used as polyvinyl alcohol.

Synthesis of the Polyamide Containing a Tertiary Amine Group

Into an autoclave were loaded ε-caprolactam (50 parts by weight), N,N-di (γ-aminopropyl) piperazine adipate (40 parts by weight),3-bis-aminomethyl cyclohexane adipate (10 parts by weight) and water(100 parts by weight), and after having been substituted with nitrogen,the autoclave was tightly closed and gradually heated. From the timewhen the inner pressure had reached 10 kg/m³, the water was distilledoff until the pressure was no longer maintained, and this was returnedto normal pressure in about 2 hours, and then allowed to react with oneanother at normal pressure for one hour. The maximum polymerizationreaction temperature was 255° C. By these processes, polyamidecontaining a tertiary amine group having a melting point of 137° C. witha specific viscosity of 1.96 was obtained.

Synthesis of the Polyamide Containing an Ether Group

Acrylonitrile was added to each of the two terminals of polyethyleneglycol having a number-average molecular weight of 600, and this wassubjected to a reduction treatment by hydrogen so that α,ω-diaminopolyoxyethylene was obtained, and an equimolar salt betweenthis and diadipic acid (60 parts by weight), ε-caprolactam (20 parts byweight) and an equimolar salt between hexamethylene diamine and adipicacid (20 parts by weight) were fused and polymerized so that polyamidecontaining an ether group having a relative viscosity of 2.50 (viscosityobtained by measuring at 25° C. a solution prepared by dissolving 1 g ofthe polymer in 100 ml of water chloral) was obtained.

Synthesis of the Polyamide Containing a Quaternary Ammonium Salt Group

The polyamide containing a tertiary amine group (100 g), synthesized asdescribed above, was dissolved in 1000 cc of methanol. Successively,adipic acid (4.4 g) was added to this. After having been stirred at 30°C. for 3 hours, methanol was distilled off so that polyamide containinga quaternary ammonium salt group was obtained.

Synthesis of the Polyamide Containing a Sulfonic Group

Into an autoclave were loaded ε-caprolactam (4270 g), hexamethylene diammonium adipate (2990 g), dimethylisophthalate (1397 g), dimethyl5-sodium sulfoisophthalate (1397 g), hexamethylene diamine (1253 g) andwater (2500 g), and this was subjected to -polycondensation at 130° C.to 270 C for 10 hours so that polyamide containing a sulfonic group wasobtained.

Preparation of the Heat Sensitive Mask Layer Coating Solution

In accordance with the composition (weight ratio) described in the heatsensitive mask layer composition of Table 1, a dispersion binder wasdissolved in a solvent, and carbon black was dispersed in this solutionto prepare a dispersion solution so that a heat sensitive mask layercoating solution was prepared. Here, in Examples 1 to 6 and ComparativeExamples 1 to 3, the solvent used here was a mixed solution of methanoland ethanol at a weight ratio of 70:30, in Comparative Example 4, thesolvent used here was a mixed solution of n-butanol and toluene at aweight ratio of 80:20, and in Comparative Example 5, the solvent usedhere was water.

Formation of the Heat Sensitive Mask Layer

The heat sensitive mask layer coating solution was applied to a PET filmsupporting member (E5000, thickness: 100 μm, made by Toyobo Co. , Ltd.)subjected to a releasing treatment on its two surfaces, by using a barcoater properly selected so as to provide a layer thickness of 1.5 μm,and this was subjected to a drying process at 120° C. for 5 minutes sothat a heat sensitive mask layer was formed.

Performance Evaluations

Performances of each of the heat sensitive mask layers thus obtainedwere evaluated in the following manner.

Light blocking effect: The optical density of the heat sensitive masklayer formed on the PET film supporting member was measured by using awhite/black transmission densitometer DM-520 (made by Dainippon ScreenMFG Co., Ltd.).

Durability: The heat sensitive mask layer formed on the PET filmsupporting member was cut into a square piece having a size of20cm×20cm, and another PET film (E5000, thickness: 100 μm, made byToyobo Co., Ltd.) was superposed on the layer surface, and while thisstate is being maintained, these were rubbed against each other once ineach of right and left directions without applying any force; then,scratches formed on the surface of the heat sensitive mask layer wereinspected by using a 10-fold loupe.

◯: No scratches

Δ:One to four scratches, each having a length of 50 μm or more, wereobserved.

x: Five or more scratches, each having a length of 50 μm or more, wereobserved.

Table 1 shows the results of these performance evaluations.

TABLE 1 The heat sensitive mask layer composition of Examples 1 to 6 andComparative Examples 1 to 5 and the results of evaluations. ExampleComparative Example 1 2 3 4 5 6 1 2 3 4 5 heat carbon black 34 34 48 3434 34 34 34 34 34 34 sensitive dispersion butyral resin 27 19 22 27 2727 66 27 mask layer binder polyamide containing a tertiary 39 47 30 66composition amine group polyamide containing an ether 39 group polyamidecontaining a quaternary 39 ammonium salt group polyamide containing asulfonic 39 group polyamide containing no polar 36 66 group polyvinylalcohol 66 results of light blocking effect 2.4 2.3 2.8 2.4 2.4 2.1 1.72.3 1.6 1.5 2.3 evaluations durability ∘ ∘ Δ ∘ ∘ ∘ ∘ x ∘ ∘ x

As clearly indicated by Table 1, in Examples 1 to 6 in which butyralresin and polyamide containing polar group were used in combination as adispersion binder for carbon black of a heat sensitive mask layer,superior properties were obtained in both of light blocking effect anddurability, although a thin film having a film thickness of 1.5 μm wasused. Here, in the case of Comparative Example 3 in which a butyralresin and polyamide containing no polar group were used in combination,the light blocking effect was extremely lowered, although the durabilitywas superior. For this reason, in the case of a printing original plateusing the heat sensitive mask layer of Comparative Example 3, the layerthickness of the heat sensitive mask layer has to be made thicker so asto allow the heat sensitive mask layer to have a light blocking effectin such a level as to serve as a negative film. As a result, it becomesnecessary to provide a high abrasion energy, and when the photosensitiveresin layer in the printing original plate is flexible, wrinkles tend tooccur on the photosensitive resin layer. Moreover, in the case ofComparative Example 2 in which a butyral resin is used alone as thedispersion binder and in the case of Comparative Example 5 in whichpolyvinyl alcohol is used alone as the dispersion binder, the durabilityis lowered although the light blocking effect is superior. Moreover, inthe case of Comparative Example 1 in which polyamide containing polargroup is used alone as the dispersion binder and in the case ofComparative Example 4 in which polyamide containing no polar group isused alone as the dispersion binder, the light blocking effect islowered although the durability is superior. Therefore, in the case of aprinting original plate using each of the heat sensitive mask layers ofComparative Examples 1 and 4, the layer thickness of the heat sensitivemask layer needs to be made thicker in the same manner as in theprinting original plate using the heat sensitive mask layer ofComparative Example 3, resulting in problems of high abrasion energy andwrinkles on the photosensitive layer.

As indicated by the above-mentioned results, it is considered that byusing a butyral resin and polyamide containing polar group incombination as a dispersion binder for carbon black of a heat sensitivemask layer, it becomes possible to obtain a printing original platehaving a heat sensitive mask layer having high light blocking effect anddurability yet prepared as a thin film.

INDUSTRIAL APPLICABILITY

Since the photosensitive letterpress printing original plate of thepresent invention has high light blocking effect and durability althoughit has a heat sensitive mask layer with a thin thickness, it iseffectively useful, in particular, for a CTP plate that is subjected toabrasion by an infra-red laser.

1. A photosensitive letterpress printing original plate including atleast (A) a supporting member, (B) a photosensitive resin layer, and (C)a heat sensitive mask layer that are successively laminated, wherein theheat sensitive mask layer (C) contains carbon black and , as adispersion binder therefor, a butyral resin as well as polyamidecontaining polar group selected from the group consisting of polyamidecontaining a tertiary amine group, polyamide containing a quaternaryammonium salt group, polyamide containing an ether group and polyamidecontaining a sulfonic group.
 2. The photosensitive letterpress printingoriginal plate according to claim 1, wherein the butyral resin and thepolyamide containing polar group have a weight ratio in a range from 20to 80:20 to
 80. 3. The photosensitive letterpress printing originalplate according to claim 1 or 2, wherein the carbon black and dispersionbinder have a weight ratio in a range from 25 to 50:20 to 75.